<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .bg {
        position: fixed;
        top: 0;
        bottom: 0;
        left: 0;
        right: 0;
        height: 100%;
        min-height: 100%;
        max-height: 100%;
        z-index: -30;
        background-image: url();
        background-repeat: no-repeat;
        background-size: cover;
        animation: animatedBackground 1000s linear infinite;
      }
      .bg:after {
        content: '';
        position: fixed;
        top: 0;
        bottom: 0;
        left: 0;
        right: 0;
        z-index: -20;
        opacity: 0.25;

        height: 100%;
        min-height: 100%;
        max-height: 100%;
        width: 700%;
        background-image: url();
        background-position: center bottom;

        animation: animatedBk 1500s linear infinite;
        z-index: -10;
      }

      .bg:before {
        content: '';

        position: absolute;
        top: 0;
        bottom: 0;
        left: 0;
        right: 0;
        height: 100%;
        min-height: 100%;
        max-height: 100%;
        width: 700%;
        opacity: 0.4;

        background-image: url();
        background-position: center bottom;

        animation: animatedBackground 1200s linear infinite;
        z-index: -10;
      }

      @keyframes animatedBackground {
        0% {
          background-position: 0 100%;
        }
        50% {
          background-position: 100% 100%;
        }
        100% {
          background-position: 0% 100%;
        }
      }

      @keyframes animatedBk {
        0% {
          background-position: 100% 100%;
        }
        50% {
          background-position: 0% 100%;
        }
        100% {
          background-position: 100% 100%;
        }
      }
    </style>
  </head>
  <body>
    <div class="bg"></div>
  </body>
</html>
